Preparation and use of (r),(r)-2,2&#39;-bis-methylnaltrexone

ABSTRACT

This invention relates to synthesis of (R),(R)-2,2′-bis-MNTX, as shown in Formula (I), and related methods and products.

FIELD OF THE INVENTION

The present invention relates to the synthesis of (R),(R)-2,2′-bis-methylnaltrexone and related methods and products.

BACKGROUND OF THE INVENTION

Methylnaltrexone (MNTX) is a quaternary derivative of the opioid antagonist, naltrexone. MNTX exists as a salt, for example, a bromide salt. The bromide salt of MNTX is also known in the literature as: Methylnaltrexone bromide; N-Methylnaltrexone bromide; Naltrexone methobromide; Naltrexone methyl bromide; MRZ 2663BR. MNTX was first reported in the mid-70s by Goldberg et al. as described in U.S. Pat. No. 4,176,186. It is believed that addition of the methyl group to the ring nitrogen of naltrexone forms a charged compound with greater polarity and less liposolubility than naltrexone, preventing MNTX from crossing the blood-brain barrier in humans. As a consequence, MNTX exerts its effects in the periphery rather than in the central nervous system with the advantage that it does not counteract the analgesic effects of opioids on the central nervous system.

MNTX is a chiral molecule, wherein the quaternary nitrogen can possess either the (R) or (S) configuration. For example, (R)-MNTX refers to a molecule of MNTX having (R) stereochemistry at the quaternary nitrogen, while (S)-MNTX refers to a molecule of MNTX having (S) stereochemistry at the quaternary nitrogen. All of the reported functions of MNTX described in the literature indicate it is a peripheral opioid antagonist. Some of these antagonist functions are described in U.S. Pat. Nos. 4,176,186, 4,719,215, 4,861,781, 5,102,887, 5,972,954, 6,274,591, 6,559,158, and 6,608,075, and in U.S. patent application publication numbers 2003/0022909A1, 20040266806, 20040259899 and 20050004155. These uses include reducing the side-effects of opioids without reducing the analgesic effect of opioids. Such side-effects include nausea, emesis, dysphoria, pruritus, urinary retention, bowel hypomotility, constipation, gastric hypomotility, delayed gastric emptying and immune suppression. The art discloses that MNTX not only reduces the side-effects stemming from opioid analgesic treatment but also reduces the side-effects mediated by endogenous opioids alone (or in conjunction with exogenous opioid treatment) such as gastrointestinal dysfunction including inhibition of gastric emptying, constipation, inhibition of gastrointestinal motility from any cause such as surgery, inflammation or excessive vagal stimulation and other such conditions including, but not limited to, those mentioned above. However, it is unclear from the art whether the MNTX used in these studies was a mixture of (R) and (S) stereoisomers or a single stereoisomer.

The art suggests that isolated stereoisomers of a compound sometimes may have contrasting physical and functional properties, although it is unpredictable in any particular circumstance. Quaternary narcotic antagonists exhibit such contrasting physical and functional properties, making it important to develop procedures to isolate and identify MNTX as pure (R)-MNTX or (S)-MNTX. Goldberg, et al.'s U.S. Pat. No. 4,176,186, and more recently Cantrell, et al.'s WO 2004/043964 A2 describe a protocol for the synthesis of MNTX involving the quaternization of a tertiary N-substituted morphinan alkaloid with a methylating agent. However, both Goldberg et al and Cantrell et al remain silent as to the stereoisomer(s) produced by the synthesis. Based on the method of synthesis described in each, it is unknown whether the MNTX so produced was either (R) or (S) or a mixture of both. Furthermore, (S)—N-methylnaltrexone ((S)-MNTX), in pure form, and a method of making pure (S)-MNTX had not been described in the literature. Therefore, researchers would have been unable to definitively characterize and distinguish the stereoisomer(s) obtained by the Goldberg et al or Cantrell et al synthesis in the absence of pure (S)-MNTX as a standard.

In addition to the isolation and characterization of each stereoisomer of quaternary narcotic antagonists, it can be desirable to isolate the antagonists from any additional impurities in the manufacture of a pharmaceutical composition. Generally, pharmaceutical compositions also require a high level of purity to meet regulated standards for drug quality and purity. For example, in the synthesis of MNTX as described above, impurities are often formed, including degradants or by-products of manufacture, which may hinder the therapeutic effects of MNTX and/or may be toxic if present in high enough quantity. As such, it is desirable to have the ability to determine both the stereochemical configuration and the purity of MNTX. To do this, it is important to identify, isolate, and chemically characterize impurities, which can be used in chromatographic procedures as standards to confirm the purity of MNTX.

SUMMARY OF THE INVENTION

The present invention relates to the identification, purification, and synthesis of an impurity of (R)-MNTX. It has been discovered that this compound can arise as an impurity either in the manufacturing process for (R)-MNTX or as a degradant when certain solutions of (R)-MNTX are stored under certain conditions. The compound is (R),(R)-2,2′-bis-methylnaltrexone. This compound is a salt, and will therefore have counterion(s). This compound can also exist as a zwitterion. Accordingly, to one aspect of the invention, there is provided a compound of Formula (I),

wherein X or Y is a counterion. In some embodiments, each X independently can be the same or different counterion or each X can be covalently attached to the other X. In a certain embodiments, Y can independently can be the same or different counterion. In a certain embodiment a mixture of X and Y can be present. In some embodiments, X or Y can be halide, sulfate, phosphate, sulfonate, nitrate, carboxylate, or an organic species. In a particular embodiment, X is bromide. In some embodiments, the compound is isolated with at least 0.5% purity, or at least 1% purity, or at least 5% purity, or at least 10% purity, or at least 15% purity, or at least 25% purity, or at least 50%, at least 75% purity, at least 95% purity, or, more preferably, at least 97% purity. In some embodiments, the compound is a solid. In other embodiments, the compound is packaged as a solution in a sealed bottle. In one embodiment, the compound is packaged as a solution in sealed bottle having a septum.

According to another aspect of the invention, there are provided kits including a compound of Formula (I) and indicia in or on the kit indicating that the compound is present in the kit. In some embodiments, the indicia indicates the purity of the compound. In some embodiments, the indicia indicates the chemical structure of the compound by providing a chemical formula or a structural drawing. In some embodiments, the kit includes a second compound that is (R)-methylnaltrexone, (S)-methylnaltrexone, or another compound,

Another aspect of the present invention provides methods for synthesizing (R),(R)-2,2′-bis-MNTX comprising reacting a first molecule of (R)-methylnaltrexone with a second molecule of (R)-methylnaltrexone in the presence of a catalyst and an oxidizing agent in amounts sufficient to form a compound of Formula (I). In some embodiments, the catalyst is present in 25 mol % or less, or, more preferably 10 mol % or less, relative to the amount of the first molecule of (R)-methylnaltrexone. In some embodiments, the oxidizing agent is present in a stoichiometric amount relative to the amount of the first molecule of (R)-methylnaltrexone. In one embodiment, the catalyst is ferric bromide. In one embodiment, the oxidizing agent is hydrogen peroxide. In some embodiments, the method further comprises isolating the compound as a solid. In some embodiments, the compound is isolated with at least 0.5% purity, or at least 1% purity, or at least 5% purity, or at least 10% purity, or at least 15% purity, or at least 25% purity, or at least 50%, at least 75% purity, at least 95% purity, or, more preferably, at least 97% purity. In some embodiments, the compound is a solid.

The present invention also provides methods for determining an impurity, comprising injecting a reference solution comprising a compound of Formula (I) into an HPLC column under a set of conditions to obtain a first HPLC chromatogram, wherein the amount and/or chemical identity of the compound present in the reference solution is known; injecting a sample solution comprising methylnaltrexone into the HPLC column under said set of conditions to obtain a second HPLC chromatogram; and determining the presence and/or the amount of the compound in the sample solution. In some embodiments, the reference solution is injected multiple times. In some embodiments, the determining comprises comparing retention times of peaks in the first HPLC chromatogram and peaks in the second HPLC chromatogram to determine the presence of the compound in the sample solution. In other embodiments, the determining comprises quantifying peak areas of the sample solution and peak areas of the reference solution on the HPLC chromatograms and estimating from these the amount of the compound in the sample solution. The sample solution can comprise (R)-methylnaltrexone and/or (S)-methylnaltrexone, or a mixture of (R)-methylnaltrexone and (S)-methylnaltrexone. In some embodiments, the HPLC column is a reverse phase column and the column is eluted using a mobile phase comprising water, methanol, trifluoroacetic acid, or mixtures thereof.

The present invention also provides methods for determining an impurity in a material consisting essentially of methylnaltrexone, comprising injecting into an HPLC column, in a single or series of injections, a sample solution containing the material and spiked with a reference compound having a known chemical structure of Formula (I); obtaining an HPLC chromatogram; and determining the presence and/or the amount of the compound in the material. The sample solution can comprise (R)-methylnaltrexone, or a mixture of (R)-methylnaltrexone and (S)-methylnaltrexone. In some embodiments, the HPLC column is a reverse phase column and the column is eluted using a mobile phase comprising water, methanol, trifluoroacetic acid, or mixtures thereof. The method may further comprise documenting in a written form the chemical identity of the compound and the amount of the compound as an impurity.

The present invention also provides methods for determining an impurity in a material consisting essentially of methylnaltrexone, comprising injecting, in a single or series of injections, a solution in which the material is dissolved into an HPLC column and obtaining an HPLC chromatogram; determining the amount in the material of a compound known to have the structure of Formula (I); and documenting in a written form the chemical identity of the compound and the amount of the compound as an impurity in the material. In some cases, the amount in the material of the compound is determined by (i) identifying a peak on the chromatogram that corresponds to a peak on a control chromatogram of a compound known to have the structure of Formula (I), (ii) identifying a peak on the chromatogram that corresponds to a relative retention time of a compound known to have the structure of Formula (I), and/or (iii) identifying a peak on the chromatogram that corresponds that corresponds to a known amount of a spike of the compound known to have the structure of Formula (I). The sample solution can comprise (R)-methylnaltrexone, or a mixture of (R)-methylnaltrexone and (S)-methylnaltrexone. In some embodiments, the HPLC column is a reverse phase column and the column is eluted using a mobile phase comprising water, methanol, trifluoroacetic acid, or mixtures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a synthetic scheme for the synthesis of (R),(R)-2,2′-bis-methylnaltrexone ((R),(R)-2,2′-bis-MNTX), according to one embodiment of the invention.

FIG. 2 shows an alternative synthetic scheme for the synthesis of (R),(R)-2,2′-bis-MNTX, according to one embodiment of the invention.

DETAILED DESCRIPTION

The present invention provides synthetic routes for the synthesis of (R),(R)-2,2′-bis-methylnaltrexone ((R),(R)-2,2′-bis-MNTX), and related products and methods.

(R),(R)-2,2′-Bis-MNTX has the structure in Formula (I):

wherein X or Y is a counterion. In some cases, each X independently can be the same or different counterion. Each X can be covalently attached to the other X. The counterion can be, for example, a halide (e.g., iodide, bromide, chloride or fluoride), sulfate, phosphate, nitrate, or an anionic-charged organic species (e.g., sulfonates, such as mesylate, besylate, tosylate, triflate, and the like, carboxylates, such as formate, acetate, citrate, fumarate, and the like). In one embodiment, the counterion is a halide, such as bromide.

The present invention provides methods for the synthesis of (R),(R)-2,2′-bis-MNTX. FIG. 1 shows the synthesis of (R),(R)-2,2′-bis-MNTX, according to one embodiment of the invention. The phenolic ring of a first (R)-MNTX molecule may be coupled with another phenolic ring of a second (R)-MNTX molecule in the presence of a catalyst and an oxidizing agent in amounts sufficient to produce (R),(R)-2,2′-bis-MNTX. Alternatively, the phenolic ring of a naltrexone molecule may be coupled with another phenolic ring of a second naltrexone molecule under similar coupling conditions, and the resulting naltrexone dimer may be alkylated at the N-substituted tertiary amines with a methylating agent to form (R),(R)-2,2′-bis-MNTX, as shown in FIG. 2.

Because 2,2′-bis-MNTX contains two chiral quaternary amines, it may possess a number of different stereochemical configurations (e.g., (R),(R)-2,2′-bis-MNTX, (S),(S)-2,2′-bis-MNTX, or (R),(S)-2,2′-bis-MNTX) based on the stereochemistry of the individual MNTX molecules which are coupled to form the 2,2′-bis-MNTX dimer. Methods described herein utilize MNTX with at least 99.5% in the (R) configuration as the starting material, such that the (R),(R)-2,2′-bis-MNTX isomer is the major isomer formed. While other isomers of 2,2′-bis-MNTX may possibly be formed, they would be present in very low or negligible quantities. The synthesis of pure (R)-MNTX is described in U.S. patent application Ser. No. 11/441,395, filed May 25, 2006, Publication Number US 2007-0099946 A1. The stereochemical configuration of (R)-MNTX may be determined by using pure (S)-MNTX as a reference standard. The synthesis of pure (S)-MNTX is described in U.S. patent application Ser. No. 11/441,452, filed May 25, 2006, Publication Number US 2007-0265293 A1. (R)-MNTX has been found to be an opioid antagonist, while (S)-MNTX is a partial opioid agonist.

In some cases, methods of the invention comprise a coupling reaction, such as the oxidative coupling of aryl groups (e.g., phenols). The coupling reaction can involve use of a catalyst (e.g., ferric bromide) that is capable of oxidatively catalyzing the formation of a covalent bond between two molecules, such as two aryl molecules. The catalyst may comprise a metal, such as iron. Examples of catalysts include, but are not limited to, iron salts including potassium ferrocyanide, ferric chloride, and ferric bromide, or other coupling agents known to those of ordinary skill in the art. In some embodiments, it is preferred that the catalyst be present in the reaction in a sub-stoichiometric amount (e.g., catalytic amount) relative to the substrate (e.g., MNTX, naltrexone). For example, the catalyst may be present in 25 mol % or less, more preferably 10 mol % or less, relative to the amount of MNTX or naltrexone. In one embodiment, the catalyst is ferric bromide and is present in the reaction in 10 mol %. Catalysts are well known to those of ordinary skill in the art and are described extensively in both the patent literature and in chemistry text books. Appropriate catalysts can be selected with no more than routine skill by one of ordinary skill in the art.

An oxidizing agent may also be included in the coupling reaction. In one embodiment, the oxidizing agent is hydrogen peroxide. Without wishing to be bound by theory, one possible mechanism for the coupling reaction may comprise regeneration of the catalytic species by the oxidizing agent upon formation of the dimer molecule (e.g., (R),(R)-2,2′-bis-MNTX). For example, the catalyst may be reduced to a catalytically inactive molecule upon formation of the dimer molecule. The oxidizing agent may then be used to, for example, oxidize the catalytically inactive molecule to regenerate the active form of the catalyst. In some embodiments, the oxidizing agent may be hydrogen peroxide. It may be preferred that the oxidizing agent be present in the reaction in a stoichiometric amount (e.g., one mole equivalent) relative to the substrate (e.g., MNTX, naltrexone). Optionally, oxygen may be bubbled through the reaction mixture to promote the coupling reaction.

The coupling reaction may be performed in any organic solvent, mixture of organic solvents, or mixture of organic solvent(s) and water. In some embodiments, the reaction may be performed in acidic conditions. In some embodiments, the reaction may be performed in basic conditions. In one embodiment, the coupling reaction is performed in a mixture of water, methanol, and trifluoroacetic acid. Also, the coupling reaction may be performed at any temperature between 0° C. and room temperature. In some cases, the second agent may be added to the reaction mixture at 0° C. over a period of time, and the resulting mixture is warmed to room temperature. The coupling reaction may proceed from 1 hour to about 10 hours, preferably about 1 hour to about 5 hours, more preferably about 1 to 2.5 hours, most preferably about 1.5 hours. The reaction can be monitored using high performance liquid chromatography (HPLC).

In some cases, methods of the invention may further comprise a methylation reaction, such as methylation of the N-substituted tertiary amines of a naltrexone dimer (FIG. 2). The methylation reaction can involve use of a methylating agent, such as methyl bromide. As used herein, the term “methylating agent” refers to a reactive species having electrophilic properties that is capable of introducing a methyl group at the nitrogen atom of naltrexone, so as to form a covalent bond therewith. Illustrative methylating agents can be represented by the formula CH₃Z, wherein “Z” is a leaving group which, upon its departure, enables a covalent bond to be formed between the methyl group and the nitrogen atom of naltrexone, forming MNTX. Methylating agents and leaving groups are well known to those of ordinary skill in the art and are described extensively in both the patent literature and in chemistry text books. Suitable Z groups include, but are not limited to, fluoro, chloro, bromo, iodo, —OSO₂CF₃, CH₃OSO₂O—, —OSO₂CH₃, —OSO₂C₆H₄-p-CH₃, —OSO₂C₆H₄-p-Br.

The method can further involve purification of (R),(R)-2,2′-bis-MNTX using at least one purification technique, such as chromatography or crystallization. The chromatography can be reverse-phase chromatography, regular-phase chromatography, and/or ion exchange chromatography. In some embodiments, the regular-phase chromatography may involve use of an alumina or silica gel column. In some cases, methods of the invention may involve use of a reverse phase HPLC column followed by ion exchange chromatography. For reverse phase chromatography, the HPLC column may be eluted using a mobile phase comprising water, methanol, trifluoroacetic acid, or mixtures thereof. The crystallization solvent can be an organic solvent, a mixture of organic solvents, or a mixture of organic solvent(s) plus water. A preferred solvent can be an alcohol, such as methanol. Methods for both chromatography and crystallization are known to those of ordinary skill in the art.

In some cases, the HPLC column is a reverse phase column and the column is eluted using a mobile phase comprising water, methanol, trifluoroacetic acid, or mixtures thereof.

In an illustrative embodiment, the coupling reaction was carried out with a catalytic amount of ferric bromide and a stoichiometric amount of hydrogen peroxide. The procedure was carried out with 8.5 g of (R)-MNTX to produce 660 mg of >97% pure product in 7.8% yield after reverse phase chromatographic separation, followed by ion exchange chromatography and crystallization from methanol, as described more fully below. It should be understood that one of ordinary skill in the art can optimize the methodologies described herein to obtain higher purity and/or higher yield of (R),(R)-2,2′-bis-MNTX, in which the crystallized product is greater than 99%, or even greater than 99.9%, pure. The relative retention times for (R),(R)-2,2′-bis-MNTX, (R)-MNTX and (S)-MNTX are 1.55, 1.00, and 0.89, respectively, as determined by HPLC Method B, described herein.

Where purity of a solid is concerned, purity is the weight percent of the (R),(R)-2,2′-bis-MNTX versus that of other compounds in the solid. Where purity of a solution is concerned, purity is the weight percent of the (R),(R)-2,2′-bis-MNTX versus that of other compounds in the solution. Purity may be determined by HPLC, wherein the percent purity of a compound refers to the weight percent of the compound present in solution relative to the weight percent of other compounds present in the same solution.

The present invention may advantageously provide (R),(R)-2,2′-bis-MNTX in sufficient purity in order to enable its use as a reference or standard in various analytical methods (e.g., HPLC), as described more fully below. In some embodiments, (R),(R)-2,2′-bis-MNTX may be isolated with at least 0.5% purity, at least 1% purity, at least 5% purity, at least 10% purity, at least 15% purity, at least 25% purity, at least 50% purity, at least 75% purity, at least 95% purity, more preferably, with at least 97% purity. In some embodiments, (R),(R)-2,2′-bis-MNTX may be isolated and/or packaged as a solid. In some embodiments, (R),(R)-2,2′-bis-MNTX can be packaged as a solution in a container such as a sealed bottle or vial, or a sealed bottle comprising a septum.

In another aspect, the present invention provides methods for determining the presence and/or amount of (R),(R)-2,2′-bis-MNTX in a sample (e.g., a sample comprising (R)-MNTX). For example, (R),(R)-2,2′-bis-MNTX may be formed as an impurity during the synthesis of (R)-MNTX. As used herein, the term “impurity” may refer to degradants which arise during storage of (R)-MNTX and/or by-products formed in a chemical reaction for manufacturing of (R)-MNTX. In one embodiment, the method comprises injecting a reference solution comprising (R),(R)-2,2′-bis-MNTX into an HPLC column under a set of conditions to obtain a first HPLC chromatogram wherein the amount and/or chemical identity of (R),(R)-2,2′-bis-MNTX present in the reference solution is known, injecting a sample solution comprising MNTX into the HPLC column under the same set of conditions to obtain a second HPLC chromatogram, and comparing the first HPLC chromatogram with the second HPLC chromatogram to determine the presence and/or amount of the impurity. The reference solution may be formed by dissolving a sample (e.g., solid sample) of (R),(R)-2,2′-bis-MNTX in a first solvent, and the sample solution may be formed by dissolving a solid sample in a second solvent. In some embodiments, the reference solution may contain an additional compound(s), wherein the amount and/or identity of the additional compound(s) is also known. In one embodiment, the sample (e.g., sample solution) may comprise (R)-MNTX. It should be understood that the invention may encompass other samples suspected of containing (R),(R)-2,2′-bis-MNTX.

In one embodiment, the presence of (R),(R)-2,2′-bis-MNTX in the sample solution may be determined by comparing retention times of peaks in the first HPLC chromatogram with the retention times of peaks in the second HPLC chromatogram. For example, the standard solution comprising (R),(R)-2,2′-bis-MNTX may produce a chromatogram with a peak corresponding to (R),(R)-2,2′-bis-MNTX and having a particular retention time. A sample solution may then be injected into the HPLC column under the same conditions as the standard solution, and the resulting chromatogram may be studied to determine if a peak exists at the same retention time as the peak corresponding to (R),(R)-2,2′-bis-MNTX in the HPLC chromatogram of the standard solution. The existence of such a peak can indicate that (R),(R)-2,2′-bis-MNTX is present in the sample. In another embodiment, the amount of (R),(R)-2,2′-bis-MNTX in the sample solution may be determined by comparing the area of peaks in the first HPLC chromatogram with the area of peaks in the second HPLC chromatogram, and calculating from these the content of (R),(R)-2,2′-bis-MNTX in the sample solution.

In some embodiments, the present invention provides methods for determining an impurity in a material consisting essentially of methylnaltrexone, where a sample solution containing the material and spiked with a reference compound having a known chemical structure of Formula (I), as described herein, is injected into an HPLC column and an HPLC chromatogram is obtained to determine the presence and/or the amount of the compound in the material.

Methods of the invention may further comprise documenting in a written form the chemical identity of the compound and the amount of the compound as an impurity in the material.

In other embodiments, the present invention provides methods for determining an impurity in a material consisting essentially of methylnaltrexone, wherein a solution in which the material is dissolved is injected into an HPLC column and an HPLC chromatogram is obtained to determine the amount in the material of a compound known to have the structure of Formula (I), as described herein. The chemical identity of the compound and the amount of the compound as an impurity in the material may then be documented, in a written form. The amount in the material of the compound may be determined by (i) identifying a peak on the chromatogram that corresponds to a peak on a control chromatogram, (ii) identifying a peak on the chromatogram that corresponds to a relative retention time of a compound known to have the structure of Formula (I), and/or (iii) identifying a peak on the chromatogram that corresponds that corresponds to a known amount of a spike of the compound known to have the structure of Formula (I).

Some embodiments of the invention may be useful in determining the amount and/or presence of (R),(R)-2,2′-bis-MNTX in a sample comprising (R)-MNTX. The sample may be a sample of freshly manufactured material or the sample may be one stored for a defined period of time. In one embodiment, a sample of (R)-MNTX may be stored and periodically analyzed using methods described herein to determine the presence and/or amount of (R),(R)-2,2′-bis-MNTX in the sample which may have been formed by, for example, degradation of (R)-MNTX. In some cases, the sample may be placed in conditions to intentionally promote degradation of (R)-MNTX, wherein the sample is periodically analyzed using methods described herein to determine the presence and/or amount of (R),(R)-2,2′-bis-MNTX in the sample. The sample solution may also comprise (R)-MNTX, (S)-MNTX, or mixtures thereof. In one embodiment, the sample solution comprises (R)-MNTX and/or (S)-MNTX. In another embodiment, the sample solution comprises a mixture of (R)-MNTX and (S)-MNTX.

In another aspect of the invention, kits are provided containing (R),(R)-2,2′-bis-MNTX. For example, a kit including (R),(R)-2,2′-bis-MNTX may be provided for the purpose of producing standards for analytical methods, such as HPLC. In one embodiment, the kit may comprise (R),(R)-2,2′-bis-MNTX and indicia in or on the kit indicating that (R),(R)-2,2′-bis-MNTX is present in the kit. In some cases, the indicia indicates the purity of (R),(R)-2,2′-bis-MNTX. The indicia may also indicate the chemical structure of the compound by providing a chemical formula or a structural drawing. In some cases, the kit can include a second compound that is (R)-MNTX, (S)-MNTX. or another compound The indicia may indicate that the second compound is present in the kit, or may indicate the purity of the second compound or the chemical structure of the second compound by providing a chemical formula or a structural drawing. It should be understood that the kit may also include additional compounds, similarly identified by the indicia.

In some cases, the kit comprises a solution of (R),(R)-2,2′-bis-MNTX. In some cases, the kit comprises a solid sample of (R),(R)-2,2′-bis-MNTX. A “kit,” as used herein, typically defines a package or an assembly including one or more of the compositions of the invention, and/or other materials associated with the invention, such as solvents. Each of the compositions of the kit may be provided in liquid form (e.g., in solution), or in solid form. In certain cases, some of the compositions may be constitutable or otherwise processable, for example, by the addition of a suitable solvent or other species, which may or may not be provided with the kit. Examples of other compositions or components associated with the invention include, but are not limited to, solvents, surfactants, diluents, salts, buffers, emulsifiers, chelating agents, antioxidants, drying agents, needles, syringes, packaging materials, tubes, bottles, flasks, beakers, dishes, frits, filters, rings, clamps, wraps, patches, containers, and the like, for example, for using, modifying, assembling, storing, packaging, preparing, mixing, diluting, and/or preserving the compositions components for a particular use, for example, to a sample.

A kit of the invention may, in some cases, include instructions in any form that are provided in connection with the compositions of the invention in such a manner that one of ordinary skill in the art would recognize that the instructions are to be associated with the compositions of the invention. For instance, the instructions may include instructions for the use, modification, mixing, diluting, preserving, assembly, storage, packaging, and/or preparation of the compositions and/or other compositions associated with the kit. In some cases, the instructions may also include instructions for the delivery of the compositions, for example, for a particular use, e.g., to a sample. The instructions may be provided in any form recognizable by one of ordinary skill in the art as a suitable vehicle for containing such instructions, for example, written or published, verbal, audible (e.g., telephonic), digital, optical, visual (e.g., videotape, DVD, etc.) or electronic communications (including Internet or web-based communications), provided in any manner.

As used herein, “instructions” can define a component of instructional utility (e.g., directions, guides, warnings, labels, notes, FAQs or “frequently asked questions,” etc.), and typically involve written instructions on or associated with the invention and/or with the packaging of the compounds of the invention. Instructions can also include instructional communications in any form (e.g., oral, electronic, audible, digital, optical, visual, etc.), provided in any manner such that a user will clearly recognize that the instructions are to be associated with compounds of the invention.

EXAMPLES Example 1 Synthesis of (R),(R)-2,2′-bis-MNTX

A solution of (R)-MNTX (8.5 g, 0.0195 mol) in 40 mL of water:methanol:trifluoroacetic acid (94.9:5:0.1) and ferric bromide (0.57 g, 1.95 mmol) was cooled in an ice/water bath. A solution of hydrogen peroxide (0.66 g, 0.0195 mol) in 10 mL of water was added to the solution over a period of 30 minutes. After stirring for 30 minutes in the ice bath, the solution contained 74% (R)-MNTX and 16% (R),(R)-2,2′-bis-MNTX by HPLC analysis. The solution was removed from the ice bath and was warmed to room temperature. After 30 min, HPLC analysis indicated that the solution contained 44% (R)-MNTX, 27% (R),(R)-2,2′-bis-MNTX, and 2% of an impurity at longer retention time. After one additional hour, HPLC analysis indicated that the solution contained 27% (R)-MNTX, 35% (R),(R)-2,2′-bis-MNTX, and 6.2% of the impurity, plus additional impurities.

Example 2 Purification of (R),(R)-2,2′-bis-MNTX

The reaction mixture was loaded directly onto a preparative reverse phase column (Biotage 40M C18) and eluted with 2.4 L of a linear solvent gradient of water:methanol, with a constant concentration of TFA (95:5 water:methanol to 82.5:17.5 water:methanol, in 12 mL fractions). The fractions were analyzed by HPLC and fractions of similar composition were combined in two lots according to purity. The solvents were removed by rotary evaporation under aspirator pressure and then under high vacuum while maintaining a bath temperature below 34° C.

Both lots were subsequently purified separately by reverse phase chromatography on the Biotage 40M C18 column with 2.4 L of a linear gradient of 95:5:0.1 to 70:30:0.1 water:methanol:TFA. The fractions were analyzed by HPLC and combined in two lots according to purity. The solvents were removed from the impure fractions by rotary evaporation under aspirator pressure and then under high vacuum while maintaining a bath temperature below 34° C. and resubmitted to chromatography as described above, for a total of four iterations of chromatography.

The pure fractions were combined and the solvent was removed as described above to give 1.0 g of (R),(R)-2,2′-bis-MNTX as a yellow glass. A Bio-Rad AG-1-X8 ion exchange column (60 g) was prepared by eluting with 500 mL of 1M HBr and then 4 L of distilled water until pH of the eluant was approximately 6. The product was dissolved in 15 mL of water:methanol (95:5) and was loaded onto the freshly prepared ion exchange column. The product was purified using distilled water as the eluant. The product was eluted in the first 100 mL of distilled water. The fractions were combined and the solvent was removed by rotary evaporation under aspirator pressure with the bath temperature maintained at 34° C. The resulting residue was dried on high vacuum line and was crystallized from 10 mL of methanol to give yellow crystals in 7.8% yield (660 mg, >97% pure,) after drying under high vacuum.

The product was observed to be unstable in neutral aqueous solution. When dissolved in d⁴-methanol/D₂0, the product transformed to a compound with a slightly shorter retention time. However, when stored overnight in water:methanol:TFA (94.9:5:0.1), the product remained stable.

HPLC analysis was performed on a Hewlett Packard 1100 series using HPLC Method A, described herein.

HPLC Method A:

Column: Phenomonex Intersil ODS-3 column (C18, 5μ, 150×4.6 mm) Flow rate: 1.5 mL/min Column temperature: 50° C. Detector: Diode array detector monitoring @ 280 nm Elution: Linear gradient

Time (min) % A % B % C Curve 0 0 90 10 initial 45 30 60 10 linear 45.1 0 90 10 linear 50 0 90 10 hold A = Methanol B = Water C = Water:methanol:TFA (49.5:49.5:1)

HPLC Method B:

HPLC Method B is described as “HPLC Method II” in US Publication Number US 2007-0099946 A1 (U.S. patent application Ser. No. 11/441,395). 

1. A compound of Formula (I),

wherein X or Y is a counterion.
 2. The compound of claim 1, wherein each X independently can be the same or different counterion or each X can be covalently attached to the other X.
 3. The compound of claim 1, wherein X or Y is halide, sulfate, phosphate, sulfonate, nitrate, carboxylate, or an organic species.
 4. The compound of claim 1, wherein X is bromide. 5-12. (canceled)
 13. The compound of claim 1, wherein the compound is isolated with at least 95% purity.
 14. The compound of claim 1, wherein the compound is isolated with at least 97% purity.
 15. The compound of claim 1, wherein the compound is a solid. 16-43. (canceled)
 44. A method for determining an impurity, comprising: injecting a reference solution comprising a compound of Formula (I),

wherein X or Y is a counterion, into an HPLC column under a set of conditions to obtain a first HPLC chromatogram, wherein the amount and/or chemical identity of the compound present in the reference solution is known; injecting a sample solution comprising methylnaltrexone into the HPLC column under said set of conditions to obtain a second HPLC chromatogram; and determining the presence and/or the amount of the compound in the sample solution.
 45. The method of claim 44, wherein each X independently can be the same or different counterion or each X can be covalently attached to the other X.
 46. The method of claim 44, wherein X or Y is halide, sulfate, phosphate, sulfonate, nitrate, carboxylate, or an organic species.
 47. The method of claim 44, wherein X is bromide.
 48. The method of claim 44, wherein the determining comprises comparing retention times of peaks in the first HPLC chromatogram and peaks in the second HPLC chromatogram to determine the presence of the compound in the sample solution.
 49. The method of claim 44, wherein the determining comprises quantifying peak areas of the sample solution and peak areas of the reference solution on the HPLC chromatograms and estimating from these the amount of the compound in the sample solution.
 50. The method of claim 44, wherein the sample solution comprises (R)-methylnaltrexone and/or (S)-methylnaltrexone. 51-53. (canceled)
 54. A method for determining an impurity in a material consisting essentially of methylnaltrexone, comprising: injecting into an HPLC column a sample solution containing the material and spiked with a reference compound having a known chemical structure of Formula (I),

wherein X is a counterion, obtaining an HPLC chromatogram, and determining the presence and/or the amount of the compound in the material. 55-57. (canceled)
 58. The method of claim 54, wherein the sample solution comprises (R)-methylnaltrexone. 59-60. (canceled)
 61. The method of claim 54, further comprising documenting in a written form the chemical identity of the compound and the amount of the compound as an impurity.
 62. A method for determining an impurity in a material consisting essentially of methylnaltrexone, comprising: injecting a solution in which the material is dissolved into an HPLC column and obtaining an HPLC chromatogram; determining the amount in the material of a compound known to have the structure of Formula (I),

wherein X or Y is a counterion, and documenting in a written form the chemical identity of the compound and the amount of the compound as an impurity in the material.
 63. The method of claim 62, wherein the amount in the material of the compound is determined by (i) identifying a peak on the chromatogram that corresponds to a peak on a control chromatogram, (ii) identifying a peak on the chromatogram that corresponds to a relative retention time of a compound known to have the structure of Formula (I), and/or (iii) identifying a peak on the chromatogram that corresponds that corresponds to a known amount of a spike of the compound known to have the structure of Formula (I). 64-66. (canceled)
 67. The method of claim 62, wherein the sample solution comprises (R)-methylnaltrexone. 68-69. (canceled) 